Antimicrobial susceptibility testing (ASTs) devices provide critical information that aids in selection of effective and appropriate antimicrobial therapy. Several AST devices are available from device manufacturers and they include disk diffusion assays, E-test strips (AB bioMerieux), and automated systems such as VITEK™ (bioMerieux), Phoenix (Becton Dickinson) and Microscan WALKAWAY™ (Siemens). Because of its simplicity, the disk diffusion assay is a widely used AST device.
The investigational lipoglycopeptide oritavancin exhibits an inherent capacity to bind to plastic and glass surfaces which has consequently confounded measurement of its true potency in susceptibility tests (Arhin, F F, et al. 2008. Antimicrob Agents Chemother 52:1597-603). To date, only the broth microdilution (BMD) minimal inhibitory concentration (MIC) test is a reference method accepted by the Clinical Laboratory Standards Institute (CLSI) for use in determining the susceptibility of a bacterial isolate to oritavancin (Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard, CLSI document M7-A8, 8th ed. Clinical and Laboratory Standards Institute, Wayne, Pa. 2009; Performance Standards for Antimicrobial Susceptibility Testing; Eighteenth Informational Supplement, CLSI document M100-S18, 7th ed. Clinical and Laboratory Standards Institute, Wayne, Pa. 2008). Recent developments have demonstrated that inclusion of 0.002% polysorbate-80 (P80) during BMD MIC testing minimizes loss of oritavancin due to non-specific binding and allows for accurate determination of the MIC of a clinical isolate (Arhin, F F, et al. 2008. Antimicrob Agents Chemother 52:1597-603).
Because BMD MIC testing is technically cumbersome, other methods that simplify susceptibility testing for oritavancin would be advantageous to clinical microbiologists. Previous studies that evaluated the feasibility of an agar-based disk diffusion assay for oritavancin concluded that slow diffusion of oritavancin in agar results in zones of clearance (zone diameters) that do not meet the minimum requirements recommended by the CLSI (≧15 mm) under standard testing conditions (Sahm, D F, et al. 2003. Feasibility study to assess disk diffusion as a method for testing the susceptibility of bacterial pathogens to oritavancin. Unpublished internal report; Turnbull, B. 2007. Disk diffusion feasibility study for oritavancin susceptibility testing. Unpublished internal report).
Development of a disk diffusion assay that permits accurate determination of the susceptibility of a bacterial isolate to oritavancin, under the minimum requirements recommended by the CLSI, would greatly aid in rapid and accurate selection of therapy using oritavancin.